Subterranean deposits of natural resources such as gas, water, and crude oil are commonly recovered by drilling wellbores to tap subterranean formations or zones containing such deposits. A drilling fluid or mud is usually circulated through a wellbore as it is being drilled to cool the bit, keep the deposits confined to their respective formations during the drilling process, and return drill cuttings, i.e., small pieces of shale and/or rock generated by the drill bit, back to the surface. Drilling through subterranean zones containing clay and shale that swell upon exposure to water requires the use of non-aqueous drilling fluids to avoid problems such as sloughing and well collapse. Such non-aqueous fluids include a base fluid such as diesel oil, mineral oil, an olefin, an organic ester, or a synthetic fluid. The drilling fluid is commonly an invert emulsion, i.e., a water-in-oil emulsion.
Solid particles are often added to non-aqueous drilling fluids for various reasons. For example, weighting agents such as barite particles may be added to the drilling fluid to increase the density of the fluid and thus ensure that the fluid provides high hydrostatic pressures in the wellbore. Unfortunately, the solid particles, particularly the relatively heavy particles of a weighting agent, may settle and/or stratify in the fluid as it is being pumped through the wellbore. As a result of such settling, a problem known as sag can occur in which the specific gravity of the drilling fluid along the fluid column varies.
Traditionally, materials such as organophilic clays have been added to drilling fluids to overcome the sag problem. However, those materials undesirably increase the viscosity of the drilling fluid and thus cause the drilling efficiency to decrease. As a result of the increase in viscosity, relatively high pump pressures may be required to convey the fluid into and out of the wellbore. Further, it may be difficult to remove drill cuttings from the wellbore, resulting in the fluid having an excessive circulation density and thus leading to the loss of its circulation in the wellbore. A need therefore exists to develop a method of reducing the sag in a non-aqueous fluid such as a drilling fluid without significantly increasing the viscosity of the fluid.